Ethylene/polar comonomer copolymers such as copolymers of ethylene and vinyl acetate and ethylene and ethyl acrylate are in general use in various forms such as film and molded articles. The applications to which these copolymers can be put, however, are rather limited because of their relatively poor mechanical strength and heat resistance despite attempts to improve these properties by crosslinking the copolymers with organic peroxides.